Although it might be expected that the products of high temperature aluminous ore chlorination would be easily separable by differential condensation because of the large differences in boiling points of the product chlorides (iron, aluminum, silicon and titanium chloride), many difficulties are apparent to those who have attempted such separations. In fact, it has been conventional practice since before World War II to chlorinate Bayer process alumina product and not bauxite or some other less expensive aluminous ore to make aluminum chloride which can be used in reduction processes that convert aluminum chloride to aluminum metal.
Thus, the expense of the Bayer process is increased in order to purify the aluminum values in a stage preceeding chlorination. Even though the Bayer process is an established technology with a high tonnage base, the cost of this additional processing greatly reduces the potential savings of a chloride electrolysis process.
Considerable experimental effort, has been expended to test the possibility of using staged condensation to remove and reduce the impurity chlorides to a satisfactory level (J. Caby, Purification du Chlorure D'aluminium anhydry, diss. ethzurich, from NR. 3631-1965). This effort has not been successful even at laboratory scale. The existence of high temperature chloride vapor complexes between aluminum trichloride and various dichlorides has recently been discovered. Complexes between iron and aluminum trichloride are also now known as similar complexes with ferrous chloride. These complexes apparently defeat attempts to purify aluminum chloride by simple partial condensation. It has been proposed that rectification of aluminum chloride captured from the gas phase by a number of solvent complexing agents be used to purify aluminum trichloride for use in cells but this technique inevitably involves heat transfer into a highly corrosive liquid, AlCl.sub.3. Even small amounts of aluminum trichloride in titanium tetrachloride will attack stainless steels, nickel and nickel-bearing alloys at highly unacceptable corrosion rates. This corrosivity has been known in the titanium chloride pigment industry for three decades.
Similarly, ferric chloride in solution in a sodium chloride complex is highly corrosive of stainless steels at tempertures near 250.degree. C., and the aluminum chloride analogue would also be expected to corrode any metallic surface through which heat was being transferred, particularly when the aluminum chloride is present in amounts above the 1/1 ratio which forms the very stable complex NaAlCl.sub.3. Thus any attempt at rectification must bring heat transfer surfaces into unacceptable corrosion conditions.
The literature is replete with attempts to partially chlorinate aluminous ores in order to reduce the impurity content to acceptable cell levels. All these attempts have fallen well short of producing a material which is usable in an alumina reduction cell, even short of producing a feed to a second chlorination step which would be low enough in impurities to make an acceptable aluminum chloride.
Attempts to treat the gases from chlorination of aluminous ores before condensation have involved the use of reducing agents, particularly the use of aluminum metal. These efforts bring the added disadvantage of a costly reducing agent, the Al product itself. Experimental work has again not indicated that impurity separation by using reducing agents is particularly successful when faced with other than the inevitable iron chloride impurities.
Put succintly, the chlorination of aluminous ores to make aluminum chloride for cell reduction to aluminum metal has been obviously desirable from an economic standpoint, but the purification problem has eluded straightforward approaches to the extent that the economic penalty of adding Bayer processing has been incurred and had to be accepted.
Now, with the aluminum ore sources of good Bayer bauxite more expensive, less available, and more subject to political intervention, other aluminous ores are being considered. These new processes are generally even more expensive than present Bayer alumina processing. Conversely, the processing of these other aluminous ores and low grade bauxites unsuitable for Bayer processing is becoming more attractive by chlorination than by other means.
This brings to a focus the necessity for a good process to produce an acceptable pure aluminum chloride cell feed by a process which eliminates the need for a Bayer process step or the production of an alumina starting material produced by an even more expensive process.